1. Field of the Invention
The present invention relates to a spin valve sensor with a biasing layer and, more particularly, to a biasing layer which produces a demagnetizing field which supports a demagnetizing field from a pinned layer structure in opposing a sense current field at a free layer structure in the spin valve sensor.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The transfer curve of a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. Readback asymmetry is defined as             V      1        -          V      2            max    ⁡          (                        V          1                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  V          2                    )      
For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in some applications, +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demagnetizing (demag) field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer, a net image current field HIM from the first and second shield layers.
A net image current field HIM is obtained by offsetting the spin valve sensor between the first and second read gap layers. For instance, if the spin valve sensor is offset closer to the second shield layer with a thinner second read gap layer the image current field HIM from the second shield layer will be greater than the net image current field HIM from the first shield layer which causes the aforementioned net image current field HIM on the free layer structure. Unfortunately, a thin second read gap layer increases the risk of shorting between the lead layers and the second shield layer. In order to overcome this problem the thickness of the second read gap layer can be increased. Unfortunately, this increases the read gap between the first and second shield layers which reduces the linear bit density of the read head.
In order to reduce demagnetizing field from the pinned layer on the free layer, the pinned layer may be an antiparallel (AP) pinned layer structure. An AP pinned layer structure has an antiparallel coupling (APC) layer which is located between ferromagnetic first and second AP pinned layers. The first and second AP pinned layers have magnetic moments which are antiparallel with respect to one another because of a strong antiferromagnetic coupling therebetween. The AP pinned layer structure is fully described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein. Because of the partial flux closure between the first and second AP pinned layers only a small net demagnetizing field is exerted on the free layer. Because of the small demagnetizing field the exchange coupling between the AP pinned layer structure and the pinning layer is increased for promoting high stability of the spin valve sensor when subjected to unwanted magnetic fields in the presence of elevated temperatures.
Unfortunately, a small demagnetizing field from an AP pinned layer structure makes it difficult to counterbalance the strong sense current field HI from the sense current. In some spin valve sensors the ferromagnetic coupling field HFC is very small or zero. This then leaves only the net image current field HIM from a gap offset in order to provide a field at the free layer structure to support the small net demagnetizing field from the pinned layer structure to oppose the sense current field from the sense current. As stated hereinabove, it is undesirable to employ the gap offset to obtain the net image current field HIM because of the problem with shorting between the lead layers and one or more of the shield layers.
The present spin valve sensor has a biasing layer with a demagnetizing field which supports the net demagnetizing field in opposing the sense current field HI exerted on the free layer structure. With this arrangement the net demagnetizing field of the pinned layer structure and the biasing layer are parallel with respect to one another. The present invention obviates the necessity of a ferromagnetic coupling field HFC and/or a net image current field HIM to counterbalance the sense current field HI in order to properly bias the free layer structure. An aspect of the invention is that the sense current IS is fed through the spin valve sensor in a direction which causes the sense current field HI to orient the magnetic moment of the biasing layer so that the demagnetizing field of the biasing layer supports the net demagnetizing field of the pinned layer structure.
In another aspect of the invention the biasing layer is composed of a material which causes specular reflection of conduction electrons back into the mean free path of conduction electrons so as to increase the magnetoresistive coefficient dr/R. A still further aspect of the present invention is that the pinned layer structure is an antiparallel (AP) pinned layer structure which produces a small net demagnetizing field. As stated hereinabove, the AP pinned layer structure promotes high stability for the spin valve sensor. Still another aspect of the invention is that the second AP pinned layer, which interfaces the spacer layer, is magnetically thicker than the first AP pinned layer. With each of the first and second AP pinned layers composed of a cobalt based material, the thicker second AP pinned layer next to the spacer layer will promote an increase in the magnetoresistive coefficient dr/R. Accordingly, the second AP pinned layer controls the orientation of the net demagnetizing field from the AP pinned layer structure on the free layer structure.
An object of the present invention is to provide a spin valve sensor wherein a free layer therein can be properly biased without a ferromagnetic coupling field HFC and/or a net image current field HIM.
Another object is to provide a method for making the aforementioned spin valve sensor.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.